Underlying advanced digital processing systems are smaller basic digital circuit building blocks. The simple building blocks are combined and arranged in such a manner as to provide extremely fast and sophisticated processing.
A carry circuit is typically used in arithmetic units, such as adder or subtractors, to process a carry operation in order to transfer a carry signal to the following carry operation. The carry circuits can be arranged to form other devices such as accumulators which can be further expanded to such devices as direct digital synthesizers (DDS).
Existing accumulator architectures include a 4 bit adder accumulator using 2 bit adder blocks generally described in C. G. Eckroot and S. I. Long, “A GaAs 4-bit adder-accumulator circuit for direct digital synthesis,” IEEE Journal of Solid State Circuits, vol. 23, no. 2, pp. 573-580, April 1988. The Eckroot and Long design details a circuit consisting of adder, register and lookahead-carry logic.
The system uses 2-bit adder blocks which are cascadable to any 2N-bit width and forms the basis for the pipelined accumulator, wherein this is particularly useful in applications where larger bit width permits greater output resolution. The pipelined structure of the adder-accumulator allows for the expansion to wider data words while preserving high clock frequency operation.
In order to alleviate the ‘slowest’ part of the adder-accumulator designs, the cascaded architecture allows for wide bit-width accumulators without much of a speed penalty, since the frequency of operation is determined by the feedback of the sum and the setup time of the carry input. As the bit-width increases, the total number of accumulators increases linearly, while the total number of registers increases in a quadratic fashion:
            #      ⁢                          ⁢      accumulators        =          bits      2                  #      ⁢                          ⁢      registers        =                            bits          2                8            -              bits        4            
This existing logic circuit is a traditional 4-bit adder with carry and propagates carry outputs. The interconnection of the 4-bit adders provides complete lookahead carry logic by partial coupling of the 2-bit registers. The power consumption of the registers becomes a dominant factor for accumulators with large bit-widths, thus limiting commercial applications that demand lower power implementations.
In the general pipelined adder-accumulators, the circuits were complex because numerous latches were required for synchronization between stages. For adder-accumulators of 8 to 10 bits total resolution, a pipelined architecture using 2-bit adder blocks seemed to provide a reasonable compromise between circuit complexity and clock speed, with the disadvantages noted herein. Among the noted aspects of the standard design, the gate propagation delays largely determined the maximum clock frequency. For example, the gate delay for the carry logic circuit using standard two-level series-gated ECL logic requires two cascaded gates. Numerous attempts have been made to increase the processing speed in a commercially viable manner.
One improvement to the typical design individual 2-bit adder blocks which contains internal pipelining and an architecture that merges the logic and latching functions is described by T. Mathew, S. Jaganathan, D. Scott, S. Krishnan, Y. Wei, M. Urtega, M. Rodwell, and S. Long, “2-bit Adder Carry and Sum Logic Circuits Clocking at 19 GHz Clock Frequency in Transferred Substrate HBT Technology,” in Proceedings of IEEE International Conference on Indium Phosphide and Related Materials, Nara, Japan, May 2001, pp. 505-508, and T. Mathew, S. Jaganathan, D. Scott, S. Krishnan, Y. Wei, M. Urtega, M. J. W. Rodwell, and S. Long, “2-bit adder: carry and sum logic circuits at 19 GHz clock frequency in InAlAs/InGaAs HBT technology,” Electronics Letters, vol. 37, no. 19, pp. 1156-1157, September. 2001. This system was designed to increase the clock rate of the carry and sum logic circuit of a 2-bit adder.
For this 2-bit adder block, the carry blocks and sum blocks contain both logic functionality and latches, thus the clock inputs Φ1 and Φ2 control these internal latches. The left and right sides of the adder are driven by opposite clock phases, Φ1 and Φ2, resulting in the computation and latching of a full 2-bit add operation in a single clock cycle.
The modular 2-bit adder forms the basis for the pipelined accumulator. While a 4-bit accumulator is demonstrated, the 2-bit adder can be cascaded to an arbitrary 2N-bit width. This makes the adder-accumulator particularly useful in applications where the larger bit width allows for greater output resolution, such as direct digital synthesizer (DDS) applications. Additionally, the pipelined structure of the adder-accumulator allows for the expansion to wider data words while preserving high clock frequency operation.
As noted in the adder circuit, the 2-bit sum and carry operations are as follows wherein A0 and B0 are the 2 adder inputs; C0 is the carry input to the full adder; S0 is the sum logic:C1=A0·B0+A0·C0+B0·C0 C2=A1·B1+A1·C+B1·C S0=A0⊕B0⊕C0 S1=A1⊕B1⊕C1 
In order to reduce delays for the carry logic circuit using standard two-level series-gated ECL logic, which requires two cascaded gates, the AND-OR logic was realized as a single three-level series-gated ECL gate. This reduced the gate delay and somewhat improved overall performance. The clock frequency was further increased by merging the logic evaluation and latching (synchronization) resulting in a four-level series-gated structure. The Carry 1 and Sum 0 are computed on one clock phase. Carry 2 and Sum 1 are computed on the other clock phase. The full 2-bit adder is computed in a single clock cycle. There are two latches added in the design to match data phases and the latches are half of the master/save latch.
While generally useful, the carry and sum circuits typically require four series-gated levels, while registers only require two series-gated levels. Unless multiple power supplies are utilized, the extra levels translate into unnecessary power consumption in the registers. The problems associated with having multiple power supplies for the design with carry and sum circuits both requiring four series-gated levels, while registers only requiring two series-gated levels was heretofore unresolved.
The processing of numerical data is typically carried out in a digital computer and consists of numerous schemas. One example involves frequency synthesizers. The general requirements for frequency generation are to provide precise frequency control and fast response, therefore the underlying circuit design must allow for high speed efficient processing, as even minor improvements reducing the processing time for a given operation can equate to significant improvements when dealing with large number crunching operations.
While carry/majority circuits are generally known, there are also known limitations with respect to speed and power requirements. What is needed, therefore, are designs and systems for improved carry/majority circuit for applications such as high speed accumulators that will provide very fast processing. Such a system should also have low power requirements and preferably utilize fabrication techniques known in the industry and be readily integrated into higher assemblies.